Originally Posted by GreenImp
I believe this is an error. The way I understood Goodyear's publication is that in order to increase the speed by 10 MPH, the cold PSI must be increased by 10 over the "recommended PSI based on tire load"....not by increasing the PSI by 10 over the "maximum PSI listed on the sidewall". I don't have any knowledge of Power King's position on this.
Tirerack has a copy of Goodyear's publication posted here...
This is a direct quote from the Tire Rack. It's how they interpret Goodyear & Power King PDFs excess speed for ST tires.
Unless assigned a specific Speed Rating, industry standards dictate tires with the ST designation are speed rated to 65 mph (104 km/h) under normal inflation and load conditions.
However, Goodyear Marathon and Power King Towmax STR tires featuring the ST size designation may be used at speeds between 66 and 75 mph (106 and 121 km/h) by increasing their cold inflation pressure by 10 psi (69 kPa) above the recommended pressure for the rated maximum load.
Do not exceed the wheel's maximum rated pressure. If the maximum pressure for the wheel prohibits the increase of air pressure, then maximum speed must be restricted to 65 mph (104 km/h).
The cold inflation pressure must not exceed 10 psi (69 kPa) beyond the inflation specified for the maximum
load of the tire.
Increasing the inflation pressure by 10 psi (69 kPa) does not provide any additional load carrying capacity."
On edit, a footnote; All ST and LT tires are allowed to have a 10 psi increase in inflation pressure above what is displayed on their sidewalls. It's seldom used but is an option any tire
manufacturer can use and explains the use by Goodyear & Power King in this situation.